A problem with one of the shuttle Endeavour's auxiliary power units forces the setback. Among thousands of disappointed onlookers: President Obama and Rep. Gabrielle Giffords.

Reporting from Titusville, Fla., and Los Angeles — Even before the astronauts awakened to begin preparations for the Endeavour shuttle launch, their fans were ready.

They flocked from across the country to the Florida coast, setting up camping chairs and rainbow umbrellas in the dawn light or hauling canoes to the water to watch from there. Some drove 19 hours straight through tornado-ravaged land and some flew thousands of miles to see Endeavour, the second-to-last space shuttle scheduled to launch into orbit.

Liftoff was scheduled for 3:47 p.m. But about 12:20 p.m., as people waited in line for hot dogs and pulled-pork sandwiches and watched the countdown clock, a calm voice announced over a loudspeaker that the launch had been postponed.

A collective hush settled over the audience. Some groaned. Others found themselves with dozens of newly purchased souvenirs — sweatshirts, blankets and astronaut pens — marked with the date of a launch that wasn't to be.

Shortly after the announcement, NASA launch director Mike Leinbach said the problem was with two heaters on one of Endeavour's auxiliary power units. The heaters are needed prevent the shuttle's hydraulic system fuel lines from freezing so they can help maneuver the shuttle in orbit.

"Today, the orbiter's not ready to fly, and as we always say in this business, we will not fly before we're ready," he said in an interview posted on NASA's website.

Endeavour's launch will take place no earlier than Monday.

Among the throngs of people who came in to see the launch were two big names: President Obama and Rep. Gabrielle Giffords (D-Ariz.). Obama met briefly with Giffords for the first time since he saw her in a Tucson hospital just after a January shooting that nearly took her life.

Giffords was there to see Endeavour's final blastoff because her husband, Mark E. Kelly, is commander of the mission. Giffords came to Cape Canaveral on Wednesday after leaving Memorial Hermann-Texas Medical Center in Houston, where she has been undergoing rehabilitative therapy.

"Attending the launch is an opportune time for her to continue her therapy progression," Dr. Gerard Francisco, the lead doctor of the brain injury rehabilitation team, said in a statement.

Giffords had been scheduled to return to Houston shortly after the launch. There was no word on whether she would remain in Florida during the delay.

Giffords appeared to take it in stride. Her congressional staff posted her sentiments on Facebook and Twitter: "Bummed about the scrub!! But important to make sure everything on shuttle is working properly."

Others didn't take it so well. One watcher noted that it was easier for a commoner to marry a prince on Friday than it was to launch a shuttle into space.

Tim Guiteras, 11, and his father, Tom, flew to Orlando from Los Angeles on the Thursday night red-eye. They went to their hotel at 5 a.m. to take a short nap, and then drove to Titusville, a town across the water from the launchpad where crowds of people were gathering.

But when they arrived, they saw people headed to their cars. Tom Guiteras checked his iPad to find out the bad news.

"I'm really disappointed," said his son, who wants to be an astronaut and already has a space exploration badge and an astronomy badge from the Boy Scouts.

It was the second time the two had bought plane tickets to see Endeavour launch. Although the airlines wouldn't refund their old tickets, the pair decided to splurge on tickets once more to try to see one of the last launches.

Rick and Tonia Hunt of Glendale also had taken an overnight flight from Los Angeles. They were incredulous when the launch was canceled. Maybe they were just exhausted — Hunt had been awake for 27 hours straight.